The tread elements in relief on a tire tread, such as the sipes, tread blocks and grooves, play a fundamental role in tire traction or adhesion to the road both in the transverse direction and in the circumferential direction. Traction is especially critical when travelling on wet or snowy roads. For wet conditions, the grooves act as evacuation channels for trapping and evacuating water to allow the tread blocks to be in contact with the road. In addition, surface tie bars are beneficial when the tire is new, and help to ensure good handling performance.
As a tire wears, the volume of the tread decreases due to frictional contact with the road surface. As the tire wears, the volume of the tire grooves decrease. As the tire wears, it is desired that the tire bar wears as well, since the tread block height is reduced and the tie bar is no longer necessary. Eventually the tire will require replacement.
It is known in the prior art to attempt to solve the degrading tread condition through the use of sunken grooves. The sunken grooves, due to a limitation in manufacturing, are generally oriented perpendicular to the shoulder region. Molding elements are typically used to form the sunken grooves, and they are often difficult to remove from the tire once the tire has been cured. To make a sunken groove, the mold elements need to be rigid enough to penetrate the uncured or green rubber tread, but flexible enough to be extracted from the cured tire without damaging the tire. Another requirement is that the sunken groove needs to be large and thick enough to be efficient, which leads to a molding element so stiff that its extraction would damage the tire tread or the element itself would fail by a fatigue fracture. Thus it is desired to have a method and apparatus of forming sunk grooves and surface tie bars that do not have the disadvantages described above.